This invention relates to the art of hot melt pumping and dispensing apparatus, and pertains more particularly to an improved heater construction for such apparatus.
The prior art describes a number of hot melt dispensers which liquefy and dispense plastic materials which are normally solid or semi-solid at room temperature, and which materials become sufficiently plastic or liquid at elevated temperatures to permit dispensing directly from the shipping container or barrel. Commonly, such devices include an arrangement for lowering a combined heating element and pump directly into the open end of the shipping container for liquefying or melting the plastic material in the region directly below a heated platen for delivery by pump of the heated and liquefied material to a remote location. Such apparatus is shown, for example, in the U.S. Pat. Nos. of Van Haase, 2,522,652 of 1960; Hooker, 3,031,106 of 1962, Weitzel, 3,113,705 of 1963; Skonberg, 3,282,469 of 1966; Van Riper, Jr., et al, 3,412,903 of 1968; and McCreary, 3,637,111 of 1972.
Commonly, the dispensing apparatus includes a heated follower plate assembly which supports a pair of gaskets about its periphery, which carries, supports or incorporates a pump, and which carries, supports or incorporates a heater or heater segments on the bottom thereof exposed directly to the plastic material within the container, for liquefying the plastic material within is exposed directly to the heater, and for delivery of such plastic material through suitable passageways to the inlet of a pump. Either a platform may be provided by means of which the container is elevated or a mechanism may be provided by means of which the plate assembly is lowered into the container.
Commonly, such heating assemblies are provided with heating elements which present flat heated surfaces to the plastic material to be melted. While such apparatus has proven to be highly useful with material which is readily elevated to a flowable temperature, and where the demand for such material is relatively low, such heating elements have proven to be less than fully effective when applied to material which requires a higher melt temperature or when applied to installations where greater flow rates are required, or both.
Hot melt materials which are dispensed by the apparatus of this invention are an extension of the thermoplastic family of products. Commonly, higher melt index polymers are being used in blends. These materials comprise a series of families including butyl rubber, meltable rubbers, and the "low end" of common polymers such as polyethylene, acrylics, polypropylene, polyesters, and polyamides. Melt temperatures can range anywhere from the ambient up through 500.degree. F. or more, with the bulk of applications being in the 250.degree. F. to 350.degree. F. range. However, there are many materials which move more readily by the addition of temperature even though they, in themselves, are not considered in the hot melt family of products. In other words, warming the product decreases its viscosity and allows it to be handled more readily. A product which, as an example, would be impossible to pump at 60.degree. F., may very well by the type of product that when elevated to the temperature of 90.degree. F. or so would be a pumpable material.
Characteristics of the pumped material range from a sharp melting product through those products which essentially only change in viscosity having no sharply defined transition temperature between the "solid" and molten phases. Chemists blend in many types of additional compounds to provide tackiness, providing a series of compounds which are considered pressure sensitive. They behave much like the equivalent materials in the rubber based families which require the elimination of solvent for their inherent pressure sensitive characteristics.
Viscosities can range anywhere from light syrup through the butyl family which can be handled at temperatures which permit, for example, extruding a bead which retains its circular cross-section as it cools.